This invention relates to aggregated pigments having a low light scattering coefficient and a high surface area. In a more specific aspect, this invention relates to aggregated pigments which have a high surface area and a low light scattering coefficient and which are useful in coating compositions for ink jet printing media. This invention also relates to a process for the manufacture of these aggregated pigments.
This invention will be described in detail with specific reference to kaolin clay. However, this invention will be understood as applicable to other mineral compositions, such as natural calcium carbonate, precipitated calcium carbonate, calcium sulfate (normally known as gypsum), bentonite, talc, aluminum oxide and aluminum hydroxide.
Kaolin is a naturally occurring, relatively fine, white clay mineral which may be generally described as a hydrated aluminum silicate. After purification and beneficiation, kaolin is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin in paper coatings serves, for example, to improve brightness, color, gloss, smoothness, opacity, printability and uniformity of appearance of the coated paper. As a filler in paper formulations, kaolin is used to extend fiber and reduce cost, and to improve opacity, brightness and other desirable characteristics of the filled paper product.
Calcined kaolin is a particular type of kaolin and is often used in large quantities for paper manufacture. Calcined kaolin can be obtained by heating (i.e., calcining) beneficiated kaolin clay at temperatures of at least 550xc2x0 C. The calcination step dehydroxylates and converts the kaolin into a noncrystalline aluminosilicate phase or metakaolin. The term xe2x80x9cdehydroxylatesxe2x80x9d refers to the removal of structural hydroxyl groups from the kaolin in the form of water vapor. The smaller particles of the feed clay are aggregated by calcination, and this aggregation increases the original volume of the kaolin and gives the calcined kaolin a xe2x80x9cfluffyxe2x80x9d appearance. Particle aggregation increases the light scattering characteristics of the kaolin (as compared to non-calcined kaolin) and, therefore, contributes a high degree of opacity to a coated paper. In addition, calcination increases the brightness of the kaolin.
Fanselow et al. U.S. Pat. No. 3,586,523 describes calcined kaolin clays.
Calcined kaolin clay pigments (such as those marketed by Thiele Kaolin Company of Sandersville, Georgia under the trademarks KAOCAL and KAOCAL LA) are widely used in the paper industry. The high brightness of the calcined clay is partly due to the removal of organic material at elevated temperatures. The brightness can also be improved through pre-calcination beneficiation processes such as magnetic separation, froth flotation, selective flocculation and chemical leaching.
Hydrous kaolin clay is another conventional product (such as that marketed by Thiele Kaolin Company under the trademark KAOFINE 90) which is widely used in the paper industry. This particular type of kaolin has not been subjected to a calcination step.
Both hydrous and calcined clay products are useful in coating compositions for conventional printing applications such as offset, rotogravure, letterpress and flexographic. However, without substantial mechanical and/or chemical modifications, hydrous and calcined clay products are not useful in coating compositions for ink jet printing applications.
In an ink jet printing process, uniformly shaped tiny droplets of aqueous or solvent based dye solutions are ejected from a nozzle onto a substrate. There are two primary types of ink jet printing - continuous ink jet printing and drop on demand ink jet printing (DOD). The continuous ink jet is used in high speed printing such as addressing, personalization, coding and high resolution color printing such as proofing. The DOD ink jet is mainly used in home, office and wide format printing.
The thermal ink jet printer is the most common DOD ink jet currently available. In this system, ink is heated and vaporized periodically with a heating element connected to the digital data to generate bubbles. Since the volume of the ink increases during vaporization, the ink is forced out of the nozzle in the form of a drop which travels and is deposited on the paper.
The inks used in ink jet printing are commonly dilute solutions of water-soluble organic dyes. The solvent portion of these inks can be as high as 98% and is a mixture of water and high boiling point alcohols. Many of the dyes used in ink jet printing inks contain sulfonic and caboxylic acid groups. At the pH of the ink, these groups are ionized and become anionic. Once deposited on the substrate, the ink must dry quickly to avoid spreading to the adjacent printing pixel. Because of the large amount of solvent used in ink jet inks, the coating must be sufficiently absorbent to remove the solvent away from the surface so that the inks will not smear. At the same time, for sharp edge acuity, the coating must fix the dye in the ink on the surface with no lateral spreading.
There are three major requirements for pigments/coatings to provide good ink jet printing characteristics: (1) high surface area and porosity for rapid absorption of the ink liquid, (2) cationic surface charge to quickly fix or immobilize the anionic ink jet dyes on the surface of the printed substrate and (3) low light scattering which improves the color of the printed image by not diluting the colors as the ink penetrates into the printed substrate.
Currently, silica is the pigment of choice for ink jet coating. Silica enhances ink jet printing by virtue of its high surface area ( greater than 150 m2/g) and porosity. However, silica is much more expensive than conventional pigments based on kaolin or calcium carbonate. Also, silica imparts high viscosity to the coating and cannot be made down at high levels of coating solids. Because of the viscosity/solids issue with silica, the coating machines cannot be integrated with paper making machines and consequently must be operated off line. This off line situation effectively reduces the productivity of the paper mills.
Several non-silica based pigments for ink jet paper coating applications have recently been introduced. For example, heat aged precipitated calcium carbonate with a surface area of at least 60 m2/g is described in Donigan et al. U.S. Pat. No. 5,643,631. This material is claimed to reduce feathering, spreading and penetration or backside show through, as well as improve optical density, dry time and water fastness.
Chen et al. PCT International Publication No. WO/98/36029 and Chen et al. U.S. Pat. No. 6,150,289 describe a coating composition comprising 100 parts calcined clay, 5-50 parts by weight of a cationic polymer, 20-30 parts by weight polyvinyl alcohol, 30-50 parts by weight of a latex binder and 0-5 parts by weight of a cross-linking agent.
Londo et al. U.S. Pat. No. 5,997,625 describes a coating composition comprising a fine particle hydrous clay, a caustic leached calcined clay and a porous mineral (zeolite). This composition exhibits the best overall color density and color definition compared with the individual components.
For various reasons, the above described products fail to provide either cost-effectiveness or performance advantages over the conventional silica pigment. Thus, there is a need in the industry for cost-effective coating pigments having equivalent or improved printing performance and Theological characteristics over silica for ink jet printing media.
Briefly described, the present invention provides aggregated pigments which have a high surface area and low light scattering and which are useful in coating compositions for ink jet printing media. The present invention also provides a process for the manufacture of these aggregated pigments. Again, although described with regard to kaolin clay, the present invention will be understood as applicable to other mineral compositions such as natural calcium carbonate, precipitated calcium carbonate, bentonite, talc, aluminum oxide and aluminum hydroxide.
As used in this application, the term xe2x80x9chigh surface areaxe2x80x9d refers to the surface area of the pigments of this invention, and this surface area is higher (i.e., greater) than the surface area of the starting material. Likewise, the term xe2x80x9caggregatedxe2x80x9d refers to the morphology of the pigments of this invention, which are clusters of a few to several fine individual particles that are smaller than the original starting material particles. Also, xe2x80x9clow light scatteringxe2x80x9d refers to the light scattering coefficient of the pigments of this invention, and this light scattering is lower than the light scattering of the starting material. This aggregated morphology, high surface area and low light scattering enable the pigments of this invention to be useful in coating compositions for ink jet printing media.
As will be seen in greater detail below, the pigments of this invention have other characteristics which are either equivalent to or improved over the corresponding characteristics of silica.
Accordingly, an object of this invention is to provide an aggregated pigment having a high surface area and low light scattering.
Another object of this invention is to provide an aggregated pigment having a high surface area and low light scattering and which is useful in coating compositions.
Another object of this invention is to provide an aggregated pigment having a high surface area and low light scattering and which is useful in coating compositions for ink jet printing media.
Another object of this invention is to provide an aggregated kaolin clay pigment having a high surface area and low light scattering.
Another object of this invention is to provide an aggregated kaolin clay pigment having a high surface area and low light scattering and which is useful in coating compositions.
Another object of this invention is to provide an aggregated kaolin clay pigment having a high surface area and low light scattering and which is useful in coating compositions for ink jet printing media.
Another object of this invention is to provide an aggregated mineral pigment having a high surface area and low light scattering.
Another object of this invention is to provide an aggregated mineral pigment having a high surface area and low light scattering and which is useful in coating compositions.
Another object of this invention is to provide an aggregated mineral pigment having a high surface area and low light scattering and which is useful in coating compositions for ink jet printing media.
Still another object of this invention is to provide a process for the manufacture of an aggregated pigment having a high surface area and low light scattering.
Still another object of this invention is to provide a process for the manufacture of an aggregated pigment having a high surface area and low light scattering and which is useful in coating compositions.
Still another object of this invention is to provide a process for the manufacture of an aggregated pigment having a high surface area and low light scattering and which is useful in coating compositions for ink jet printing media.
Still another object of this invention is to provide a process for the manufacture of an aggregated kaolin clay pigment having a high surface area and low light scattering.
Still another object of this invention is to provide a process for the manufacture of an aggregated kaolin clay pigment having a high surface area and low light scattering and which is useful in coating compositions.
Still another object of this invention is to provide an aggregated kaolin clay pigment having a high surface area and low light scattering and which is useful in coating compositions for ink jet printing media.
Yet still another object of this invention is to provide a process for the manufacture of an aggregated mineral pigment having a high surface area and low light scattering.
Yet still another object of this invention is to provide a process for the manufacture of an aggregated mineral pigment having a high surface area and low light scattering and which is useful in coating compositions.
Yet still another object of this invention is to provide a process for the manufacture of an aggregated mineral pigment having a high surface area and low light scattering and which is useful in coating compositions for ink jet printing media.
These and other objects, features and advantages of this invention will become apparent from the following detailed description.